The present invention relates in general to a venting and filtering system for minimizing pressure differentials between the interior and exterior of a liquid storage tank while, at the same time, precluding entry of contaminants into the interior of the tank; and, more particularly, to an improved closure assembly for closing the access manhole in a liquid storage tank such, for example, as the access manhole commonly located in the top of a tank trailer of the type used to transport comestible liquid products such as milk, wherein (i) the manhole cover is provided with an opening extending vertically therethrough, (ii) an upstanding tubular wall surrounds the opening and defines a vertical passage extending through the manhole cover, (iii) a sheet of filter material is removably secured to the closure assembly and extends completely across the vertical passage in face-to-face contact with the upper edge of the tubular wall about the entire perimeter thereof, and (iv) a dome-shaped cover removably secured to the closure assembly is seated on the upstanding wall and projects rearwardly therefrom to define a downwardly facing vent, with the vent, dome-shaped cover and upstanding wall defining a tortuous path extending through the closure assembly for permitting freedom of air movement therethrough so as to minimize pressure differentials between the interior and exterior of the tank while, at the same time, spillage of the liquid contents of the tank is effectively precluded and contaminants are effectively filtered and prevented from entering into the tank; yet, wherein the tank truck operator has ease of access to the filter medium so that such filter medium can be replaced on a routine periodic basis--e.g., daily.
In the dairy industry, milk is commonly transported from dairy farms to a central processing center by means of tank trucks and trailers. The milk-containing tank itself is generally made of stainless steel and is commonly of a double-wall construction with insulation between the inner and outer stainless steel walls, so that the contained milk can be kept at an adequately low temperature while being transported.
Because of the sanitation requirements of the dairy industry and because of Federal regulations, the interior of the milk tank must be thoroughly washed at periodic intervals. Also, it is necessary to provide the tank with an access opening, commonly called a manhole, to permit an inspector to actually enter the interior of the tank. Obviously, while milk is being transported, this access opening must be properly covered both to prevent milk from being spilled out of the opening and, also, to prevent outside contaminants from entering the tank and being mixed with the milk. There is a further requirement that the interior of the tank be vented to outside atmosphere since, with an airtight tank, even small temperature variations in the milk would cause undesirable pressure differentials between the area inside the tank and the surrounding atmosphere.
Moreover, it will be appreciated by those skilled in the art that during the course of a normal operating day, the tank truck operator will make a number of stops at individual dairy farms where milk will pumped from the individual producers' storage tanks into the tank trailer and, at the end of the day, the milk will be pumped out of the trailer at a centrally located milk processing center. During such on-loading and off-loading operations, milk is being pumped into and out of the tank at relatively high rates--e.g., at rates on the order of up to 400 gallons per minute. Because of this fact, it is essential that provision be made for permitting such relatively high pumping rates without encountering significant increases and/or decreases in interior storage tank pressure, thereby precluding damage to both the storage tank and the pumping means. The venting and filtering system incorporated in the closure assembly for the tank trailer is ideally suited for this purpose.
Typical of the arrangements that have heretofore been provided for venting the interior of a milk storage tank or the like are the venting closure assemblies illustrated and described in U.S. Pat. Nos. 4,081,107 and 4,127,216 issued to Light Industrial Design Co., Inc., as the assignee of Timothy J. Martin, Jr. and David C. Waschke. Such patents illustrate, describe and claim a surge-spill preventing system marketed under the trademark "GASKOVENT" by the assignee of the aforesaid Martin et al patents and wherein a vented sealing device is mounted in the tank manhole and permits of air entry into the interior of the device at diametrically opposed peripheral points, and air movement through the device about the periphery thereof, thus defining an effective seal which prevents spillage of the liquid contents of the tank as a result of the tortuous path through the seal while permitting air movement through such tortuous path for the purpose of tending to equalize the internal tank pressure with atmospheric pressure. An opening is formed in the manhole cover and a piece of filter material is mounted in the opening for the purpose of filtering air movement through the closure assembly.
While the aforesaid prior art closure assemblies have been able to perform the closing, venting and filtering functions for a milk tank, there is a continuing need for improvement with regard to such things as simplicity of structure, ease and realiability of operation, capability of being thoroughly cleaned, low cost of initial fabrication, low cost in operation and maintenance, and ease of replacement of the filter medium on a daily basis.